Multi-input multi-output (MIMO) is known as an example of a technique realizing an increase in throughput of a wireless local area network (LAN). In MIMO, it is possible to improve a data rate (that is, frequency utilization efficiency) by sending and receiving data using many antennas on the transmitting side and the receiving side, and also it is possible to perform advanced beamforming. Beamforming is a technique that may control the amplitude and phase of a transmitted signal and a received signal at each antenna, thereby giving directivity to a transmission/reception beam and changing the shape of the beam.
In beamforming, a signal is sent so that the maximum signal power may be obtained at the receiving side (for example, terminal) after estimating on what kind of propagation path the signal transmitted by the transmitting side (for example, base station) is received on the receiving side. In order for the transmitting side to estimate the state of this propagation path, for example channel state information (CSI) is used.
CSI indicates channel state information and is measured on the receiving side based on quality information, for example, when a reference signal (RS) sent from the transmitting side is received at the receiving side. Based on the CSI fed back from the receiving side, the transmitting side calculates a channel estimation matrix having the transfer functions from the respective antennas on the transmitting side to the respective antennas on the receiving side as components to perform beamforming by using this channel estimation matrix. In this way, the transmitting side (for example, base station) may perform wireless communication by MIMO with a single receiving side (for example, terminal), or wireless communication (that is, multiple user (MU)-MIMO) by MIMO with a plurality of receiving sides (for example, terminals).
As the related art relating to sending a data signal by using beamforming in a communication system to which MIMO is applied, for example, PTL 1 has been proposed. In order to be able to transmit data signals by applying beamforming also to user terminals located away from the base station, PTL 1 discloses that a downlink reference signal (for example, channel state measurement reference signal (CSI-RS)) is transmitted to a user terminal by beamforming using a fixed weight and an appropriate weight is set by using an estimation result (CSI) from the user terminal.